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GLOSSARY

Add/drop The process where a part of the information carried in
a transmission system is demultiplexed (dropped) at an interme-
diate point and different information is multiplexed (added) for
subsequent transmission. The remaining traffic passes straight
through the multiplexer without additional processing.
Add/Drop Multiplexer (ADM) A multiplexer capable of
extracting and inserting lower-rate signals from a higher-rate
multiplexed signal without completely demultiplexing the sig-
nal.

Alarm Indicating Signal (AIS) A code sent downstream indi-
cating an upstream failure has occurred. SONET defines four
categories of AIS: Line AIS, STS Path AIS, VT Path AIS, and
DSN AIS.
All-optical network (AON) A term first used to describe the
world’s first WDM network test bed that was architected and
implemented at MIT’s Lincoln Laboratory. Today the term is
used to describe optical network environments that exploit mul-
tiple channel wavelengths for switching, routing, or distribution,
using light to the almost total exclusion of electronics.
Alternate Mark Inversion (AMI) The line-coding format in
transmission systems where successive ones (marks) are alter-
natively inverted (sent with polarity opposite that of the preced-
ing mark).

American National Standards Institute (ANSI) A member-
ship organization that develops U.S. standards and coordinates
U.S. participation in the International Standards Organization
(ISO).
Asynchronous A network where transmission system payloads
are not synchronized and each network terminal runs on its own
clock.
Asynchronous Transfer Mode (ATM) A multiplexing/switch-
ing technique in which information is organized into fixed-length
cells with each cell consisting of an identification header field

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